View source for Crystalline Structures

Different materials have different types and arrangements of [https://en.wikipedia.org/wiki/Atom '''Atoms'''] and [https://en.wikipedia.org/wiki/Molecule '''Molecules''']. The arrangement can be ordered and regular, as in the case of a [https://en.wikipedia.org/wiki/Crystal '''Crystal'''], or more random to form a [https://en.wikipedia.org/wiki/Glass '''Glass'''] or [https://en.wikipedia.org/wiki/Ceramic '''Ceramic'''].


As [https://en.wikipedia.org/wiki/Crystal '''Crystalline Solids'''] form, their atoms pack together as closely as possible - a little like bees creating a [https://en.wikipedia.org/wiki/Honeycomb '''Honeycomb'''].  The repeating patterns so formed are known as [https://en.wikipedia.org/wiki/Crystal_structure#Unit_cell '''Unit Cells'''].
[[File:Dislocations Image 005.png|500px|right|Dislocations|link=http://www.spaceflight.esa.int/impress/text/education/Mechanical%20Properties/Dislocations_02.html]]


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=====Cubic Lattices=====

[[File:CrystalineStructuresSC-3.png|200px|right|Simple Cubic Structure]]
The ordered arrangements of atoms are called [https://en.wikipedia.org/wiki/Crystal_structure '''Crystal Lattices'''] and they too can be based on hexagons - or cubes. The simplest way to arrange atoms in a cube lattice is to position one atom at each corner. This is known a [https://en.wikipedia.org/wiki/Cubic_crystal_system '''Simple Cubic (SC)'''] structure, as seen for common '''Salt'''. ''(<span style="color: green">'''Note:''' the SC lattice has eight atoms in total, each one contributing an eighth of its volume to the schematic cube as shown - therefore this particular '''Unit Cell''' holds just a single atom</span>)''


Metal atoms are packed together in one of three simple arrangements: [https://en.wikipedia.org/wiki/Cubic_crystal_system '''Body-Centred Cubic''']; [https://en.wikipedia.org/wiki/Cubic_crystal_system '''Face-Centred Cubic''']; or [https://en.wikipedia.org/wiki/Close-packing_of_equal_spheres '''Hexagonal Close-Packed''']. 
[[File:Lattices.png|500px|center|Crystal Lattices]]


[[File:CrystallineStructuresBCC-3.png|300px|right|Body Centred Cubic Structure]]
In addition to one atom at each corner, a [https://en.wikipedia.org/wiki/Cubic_crystal_system '''Body Centred Cubic'''] structure has an additional atom at the centre of the cube. The '''BCC''' lattice is the structure of '''Iron''' and '''Steel'''. ''(<span style="color: green">'''Note:''' as with the '''SC''' structure each corner atom contributes an eighth of its volume to the schematic cube, plus there is an atom at the centre, which means the  '''BCC Unit Cell''' holds two atoms</span>)''.


[[File:CrystalineStructuresFCC-3.png|300px|right|Face Centred Cubic Structure]]
In addition to one atom at each corner, a [https://en.wikipedia.org/wiki/Cubic_crystal_system '''Face Centred Cubic'''] structure has an additional atom at the centre of each side or face of the cube. The '''FCC''' lattice is the structure which '''Iron''' and '''Steel''' change to when heated beyond a certain point. ''(<span style="color: green">'''Note:''' as with the '''SC''' structure each corner atom contributes an eighth of its volume to the schematic cube, plus there is a half atom contributed on each of the six sides, which means the  '''FCC Unit Cell''' holds four atoms</span>)''.


=====Structure of Alloys=====

'''[[Alloy|Alloys]]''' are usually made by melting the main metal then stirring in a measured percentage of the additional element or elements until they dissolve - like adding salt to water. If the mixture solidifies without the additional elements separating out it is known as a [https://en.wikipedia.org/wiki/Solid_solution '''Solid Solution'''] and such '''[[Alloy|Alloys]]''' are suitable for cold working.


As the mixture cools they may form into the crystal lattices in one of two common ways:
[[File:CrystalineStructuresSolidSolution2.png|300px|right|Solid Solution Structure]]

* If the alloying elements are nearly the same size ''(i.e. no more than 15% difference)'' then one element can be substituted by the other in the lattice structure - known as '''Solid Solution Alloys''' and are the most common. In such a circumstance, there is little or no distortion to the regular crystal lattice and the '''Alloys''' ''(e.g. Brass)'' are suitable for cold working.

[[File:CrystalineStructuresInterstitial2.png|300px|right|Interstitial Carbon Structure]]
* If the contributing atoms are much smaller it is possible they can fit into the spaces between the main metal atoms. This is the case of the alloy '''Steel''' in which '''Carbon''' atoms sit in between '''Iron''' atoms. Such arrangements are known as [https://en.wikipedia.org/wiki/Interstitial_compound '''Interstitial Alloys''']. '''Iron''' will absorb about 0.015% of '''Carbon''' at room temperature to form a Solid Solution known as '''Ferrite''' but all '''Steels''' contain more than this amount of '''Carbon'''. 


[[File:CrystalineStructuresSlipPlanes2.png|300px|right|Slip Planes]]
Pure metals do not usually have sufficient '''[[Strength of Materials|Strength]]''' for most purposes. As with packing apples in a crate, if atoms are all packed together in a neat structure, layers are formed between them and one layer can slide over another along what are known as [https://en.wikipedia.org/wiki/Slip_(materials_science) '''Slip Planes'''] - this is what makes metals easy to bend,  '''[[Ductility|Ductile]]''' and '''[[Malleability|Malleable]]'''


[[File:CrystalineStructuresDistortedLattice2.png|300px|right|Distorted Lattice]]
If the atoms of different '''Alloying''' elements are of the same or similar size, an ordered and layered structure remains and the '''Slip Planes''' remain relatively smooth. But, if of differing sizes the structure may remain generally ordered but perhaps more distorted. It is these distortions in the structure, making the '''Slip Planes''' more irregular and less easy to slide over each other, that can change the physical  properties of the resulting '''Alloy'''.


=====Iron and Steel=====

Just as with adding salt to water however, there is a limit to how much of an additional element can be held in solution and, if this is exceeded, the excess separates out in the form of metallic compounds which lodge in the spaces between the main metal atoms - hence their name [https://en.wikipedia.org/wiki/Intermetallic '''Intermetallic Compounds''']. These compounds are usually very '''[[Hardness|Hard]]''' and '''[[Brittleness|Brittle]]''' and their presence results in an '''Alloy''' which generally has to be heated before it can be worked. The heating and cooling has to be carried out carefully and over a limited temperature range.


Pure [https://en.wikipedia.org/wiki/Iron '''Iron'''] can be very soft and too weak for most purposes but its properties change significantly with the addition of small amounts of impurities. '''Iron''' is extracted from [https://en.wikipedia.org/wiki/Iron_ore '''Iron Ore'''] using a [https://en.wikipedia.org/wiki/Blast_furnace '''Blast Furnace''']. This [https://en.wikipedia.org/wiki/Smelting '''Smelting'''] process inevitably contaminates the '''Iron''' and the resulting [https://en.wikipedia.org/wiki/Pig_iron '''Pig Iron'''] can contain up to 4.5% '''Carbon''' in addition to other impurities. ''(<span style="color: green">'''Note:''' The output is known as '''Pig Iron''' because, originally, the iron was cast into sand moulds fed from a common runner. Because the groups of moulds resembled a litter of suckling pigs, the individual pieces of iron were referred to as '''Pigs''' and the runner was referred to as a '''Sow'''. Nowadays, '''Pigs''' are produced by a continuous casting machine</span>)''.


The aim of [https://en.wikipedia.org/wiki/Steelmaking '''Steelmaking'''] processes such as the [https://en.wikipedia.org/wiki/Bessemer_process#Bessemer_converter '''Bessemer Converter'''] and [https://en.wikipedia.org/wiki/Open_hearth_furnace '''Open Hearth Furnace'''] ''(now superseded by [https://en.wikipedia.org/wiki/Basic_oxygen_steelmaking '''Basic Oxygen Steelmaking'''] and [https://en.wikipedia.org/wiki/Electric_arc_furnace '''Electric Arc Furnaces'''])'' is to reduce these impurities and control the amount of '''Carbon''' which remains.  


[[File:Britannica Alloys Plate Figure 11.jpg|200px|right|Britannica Alloys Plate Figure 11|link=https://en.wikipedia.org/wiki/Pearlite]]
All [https://en.wikipedia.org/wiki/Steel '''Steels'''] contain more than the very small amount of '''Carbon''' which will dissolve in '''Iron''' at room temperature. On cooling, excess '''Carbon''' separates out to form an [https://en.wikipedia.org/wiki/Intermetallic '''Intermetallic Compound''']  called [https://en.wikipedia.org/wiki/Cementite '''Cementite'''] or '''Iron Carbide'''.  In '''Steel''' which has been '''Annealed''' ''(i.e. heated to above 900<sup>0</sup>C, its [https://en.wikipedia.org/wiki/Critical_point_(thermodynamics) '''Critical Point'''], and allowed to cool slowly)'', the carbide compound can be viewed with a microscope as forming dark coloured areas layered against lighter coloured '''Ferrite''' areas. This mixture is known as [https://en.wikipedia.org/wiki/Pearlite '''Pearlite''']. 
 

[https://en.wikipedia.org/wiki/Cast_iron '''Cast iron'''] has as much as 2% to 4% of  [https://en.wikipedia.org/wiki/Carbon '''Carbon''']. If all the excess remains as [https://en.wikipedia.org/wiki/Cementite '''Cementite'''] a very '''[[Brittleness|Brittle]]''' '''White Cast iron''' is formed but more usefully, between 1% and 3% of [https://en.wikipedia.org/wiki/Silicon '''Silicon'''] is also added to the melt, which causes the excess '''Carbon''' to form [https://en.wikipedia.org/wiki/Graphite '''Graphite'''] and results in '''Grey Cast iron'''. This has improved mechanical properties and the presence of [https://en.wikipedia.org/wiki/Graphite '''Graphite'''] makes the '''Iron''' self-lubricating and a good bearing metal which can be machined ‘dry’ without the use of a cutting fluid. 


[[File:Steel 035 water quenched.png|200px|right|Steel 035 water quenched|link=https://en.wikipedia.org/wiki/Martensite]]
[https://en.wikipedia.org/wiki/Tool_steel '''Tool Steel'''] has a carbon content of between 0.5% and 1.5%. This amount of '''Carbon''' can only be held in solution with '''Iron''' at a high temperature when the '''Crystalline Structure''' changes from '''Body Centred Cubic''' to a '''Face Centred Cubic''' lattice and the dissolved '''Carbon''' forms [https://en.wikipedia.org/wiki/Austenite '''Austenite'''] - a material which only exists at these high temperatures. If the '''Steel''' is cooled or quenched suddenly, the excess '''Carbon''' is ‘frozen’ in solution to create a deformed '''BCC Crystalline Structure''' containing [https://en.wikipedia.org/wiki/Martensite '''Martensite'''] - a material too '''[[Hardness|Hard]]''' and '''[[Brittleness|Brittle]]''' for most purposes.


'''[[Heat Treatment|Tempering]]''' is a process which by controlled reheating enables the internal stress to be relieved and alters the size and distribution of carbides in the [https://en.wikipedia.org/wiki/Martensite '''Martensite'''] to improve '''[[Toughness]]''' at the expense of some '''[[Hardness]]'''.


[[Category:Materials and Components]]